With experience and age, we become more accurate at perceiving true Emotions.

http://en.wikipedia.org/wiki/Face

People are also relatively good at determining if a smile is real or fake. A recent study looked at individuals judging forced and genuine smiles. While young and elderly participants equally could tell the difference for smiling young people, the "older adult participants outperformed young adult participants in distinguishing between posed and spontaneous smiles".This suggests that with experience and age, we become more accurate at perceiving true emotions across various age groups.